In known HST travel systems, as disclosed in, e.g., JP,A 5-306768, it is general that one hydraulic pump and one hydraulic motor are connected to each other in a closed circuit and a travel unit is driven by the one hydraulic motor. In that case, the hydraulic motor is connected to front and rear wheels through a transmission and a propeller shaft such that the front and rear wheels are driven at the same time with rotation of the propeller shaft.
In another type of HST travel system, as disclosed in, e.g., JP,A 11-166623 and JP,A 11-230333, one hydraulic pump is connected in parallel to two hydraulic motors in a closed circuit and a travel unit is driven by the two hydraulic motors. In that case, the two hydraulic motors are connected to front and rear wheels through a reduction gearing and a propeller shaft such that the front and rear wheels are driven at the same time with rotation of the propeller shaft. Further, one hydraulic motor is connected to the reduction gearing through a clutch, thus enabling a low speed (high torque) mode (clutch-ON) and a high speed mode (clutch-OFF) to be switched over with ON/OFF control of the clutch. Accordingly, a transmission is not required.
Moreover, still another type of HST travel system is known in which one hydraulic pump is connected in parallel to two hydraulic motors in a closed circuit and the two hydraulic motors are connected respectively to front and rear wheels such that the front and rear wheels are driven by the separate hydraulic motors. For example, JP,A 2000-1127 discloses an HST travel system for a lawn mower which is operated in four-wheel drive with the front and rear wheels both held in a high torque state at low speed, and which is operated in a four-wheel drive mode with the front wheels held in a low torque state and the rear wheels held in a high torque state at high speed, thereby enabling a driving force to be hydraulically transmitted to the front wheels as well during travel. Further, when the lawn mower turns, it is operated in two-wheel drive with only the rear wheels driven, to thereby prevent drag of the front wheels during the turn. To change driving torque of the front-wheel hydraulic motor, a constant-ratio distribution valve is disposed at a supply/drain hydraulic port of the hydraulic pump, and a shuttle valve is disposed between main lines on the side including the front-wheel hydraulic motor. Further, a high-pressure relief valve and a low-pressure relief valve are connected in parallel on the output side of the shuttle valve.
GB2136371A discloses an HST travel system for an agricultural machine. More specifically, one of two hydraulic motors is constituted as an auxiliary motor with small displacement, the auxiliary motor is connected to front or rear wheels through a clutch and a reduction gearing, and an open/close valve is disposed between a hydraulic pump and the auxiliary motor. Connection and disconnection between the auxiliary motor and the hydraulic pump or between the auxiliary motor and the wheels are performed by switching over the clutch and the open/close valve at the same time.
Patent Document 1: JP,A 5-306768
Patent Document 2: JP,A 11-166623
Patent Document 3: JP,A 11-230333
Patent Document 4: JP,A 2000-1127
Patent Document 5: GB2136371A